8.2.3. Climate Scenarios

As discussed in IPCC (1996, WG I, Section 6.6), output from transient runs
of atmosphere-ocean general circulation models (hereafter referred to simply
as GCMs) has become available that can be used as the basis for improved regional
analysis of potential climate change. The main emphasis of current analyses
is on the simulation of seasonally averaged surface air temperature and precipitation.
Climate scenario information for North America is available from several GCMs.
In IPCC (1990, WG I), one of the five regions identified for analysis of regional
climate change simulation was central North America (35-50�N, 85-105�W). Output
for this region from different coupled model runs with dynamic oceans was analyzed
by Cubasch et al. (1994) and Kittel et al. (1998). Results for central North
America, as well as the other identified regions, are depicted in Figure
B-1 (Annex
B), which shows differences between region-average
values at the time of CO2 doubling and the control run, as well as differences
between control run averages and observations (hereafter referred to as bias)
for winter and summer surface air temperature and precipitation. These model
results reflect increasing CO2 only and do not include the effects of sulfate
aerosols. The biases in Figure B-1 (Annex
B) are presented as a reference for interpretation of the scenarios because
it can be generally expected that the better the match between control run and
observed climate (i.e., the lower the biases), the higher the confidence in
the simulated change scenarios. A summary of these transient model experiments
is given in Table B-1 (Annex
B).
Most experiments use a rate of CO2 increase of 1%/year, yielding a doubling
of CO2 after 70 years.

Scenarios produced for central North America by these transient experiments
vary quite widely among models for temperature but less so for precipitation.
GCM simulations also have been conducted that consider the effect of combined
greenhouse gas- and direct sulfate aerosol-forcing on temperature, precipitation,
and soil moisture (see Annex
B). For central North America,
the inclusion of sulfate aerosols results in a projected warming of 0-0.5�C
in the summer and 1.4-3.4�C in the winter by the year 2100. In the case of precipitation,
the inclusion of sulfate aerosol-forcing has little effect on the projections
(see Annex
B).

Using the Canadian Climate Centre (CCC) GCM (see Annex
B),
Lambert (1995) found a 4% decrease in cyclones in the Northern Hemisphere, though
the frequency of intense cyclones increased. Lambert hypothesized that the latent
heat effect is responsible for the greater number of intense storms. No change
in storm tracks was evident. A few areas showed increased frequencies, such
as off Cape Hatteras, over Hudson Bay, and west of Alaska. These results are
similar to those of Rowntree (1993), who found a 40% increase in Atlantic gales,
though fewer intense storms over eastern North America. Hall et al. (1994) and
Carnell et al. (1996) found an intensification and northward shift of storm
tracks.

Regarding sea-level rise scenarios, for IPCC Scenario IS92a, global mean sea
level is projected to be about 50 cm higher by 2100 than today, with a range
of uncertainty of 20-86 cm (IPCC 1996, WG I, Section 7.5). It is possible that
for much of the North American coastline, future sea-level rise will be greater
than the global average, given the higher historical rates of sea-level rise
along the Gulf of Mexico and Atlantic coasts (see Section
8.2.2). By contrast, future sea-level rise along the Pacific coast may be
less than the global average rise because of this region's generally lower historical
rates. Even less sea-level rise might be expected in extreme northern North
America, given the historical drop in sea levels at many locations (Titus and
Narayanan, 1996).